Searching for the
Synchrotron Cosmic Web
with the Murchison Widefield Array
Bryan Gaensler
Centre for All-sky Astrophysics / The University of Sydney
Natasha Hurley-Walker
Planelles & Quillis (2013)
The Synchrotron Cosmic Web
› Intergalactic shocks accelerate electrons and amplify magnetic fields
(Keshet et al. 2004; Hoeft & Brüggen 2007; Battaglia et al. 2009; Araya-Melo et al. 2012)
- faint synchrotron radiation should trace large-scale structure and cosmic filaments
→ direct image of large-scale structure of the Universe
→ laboratory for studying particle acceleration in low-density shocks
→ magnetic field strength of the IGM
→ direct discriminant on competing models for origin of cosmic magnetism
› Signal should dominate other radio signals on scales ~ 10′ to 1o at frequencies ~100 MHz
MHD simulation of magnetised large-scale
structure (Brüggen et al. 2005)
Injected fields vs primordial fields (Donnert, Dolag et al. 2008)
Wide-Field Radio Astronomy
Hubble
Space
Telescope
- low frequencies (30-300 MHz)
- no moving parts
MWA
- dedicated supercomputers
- enormous fields of view
SKA-low
Swinburne / LFAA consortium
› New generation of interferometers
LOFAR
© Top-Foto, Assen
MWA Consortium
MWA
The Murchison Widefield Array (MWA)
› Designated SKA precursor in outback Western Australia (Tingay et al. 2012)
- 80-300 MHz, including FM band
- 128 tiles over 3 km → ~1’ resolution
- fully operational
- proposal call every 6 months
» 1000 deg2 field of view! «
MWA Consortium
Detecting the Synchrotron Cosmic Web
› Direct detection (Bagchi et al. 2002; Wilcots 2004)
- faint emission
- Galactic foregrounds
- confusion from extragalactic point sources
Coma field at 400 MHz (Kronberg et al. 2007)
› Polarisation (Rudnick & Brown 2008)
- higher sensitivity, reduced confusion
- fainter signals, complex Galactic foregrounds
› Statistical detection (Brown et al. 2010, 2011)
- stacking
- cross-corr. with tracers of large-scale structure
2MASS galaxy distribution vs 1.4 GHz radio emission (Brown 2011)
3C 31 and NGC 315: total intensity and
diffuse polarisation (Rudnick & Brown 2008)
Radio Probes of the Thermal Cosmic Web
› Faraday rotation from background AGN
(Xu et al. 2006; Akahori & Ryu 2010; Stasyzsyn et al. 2010)
› 21cm emission from the WHIM
Xu et al. (2006)
- need to correct for foreground
Galactic Faraday rotation
(Braun 2004; Popping & Braun 2007)
- requires sensitivity to NHI < 1018 cm-2
› Dispersion of “fast radio bursts”
(Lorimer et al. 2007; Thornton et al. 2013)
Braun et al. (2004)
Thornton et al. (2013)
- need localisations & redshifts
Visualising ISM Turbulence
2+(∂U/∂x)2+(∂U/∂y)2]1/2 (Gaensler et al. 2011)
Polarisation
gradient,
|∇P|
= [(∂Q/∂x)
Linearly
Southern
polarised
Galactic
emission,
Plane
|P|Survey,
≣ (Q22+(∂Q/∂y)
+ATCA
U2)1/21.4
from
GHz
same
(Gaensler
regionet (Gaensler
al. 2001, 2011)
et al. 2001, 2011)
radio galaxies/AGN
supernova remnants
H II regions
Visualising ISM Turbulence
Observed data
Gaensler et al. (2011)
› Radio synchrotron: a key diagnostic of the cosmic web
Brüggen et al. (2005)
Summary
› New generation of wide-field low-frequency
interferometers can now search for this signal
- SKA-low in pre-construction phase
› Web-finding tools may have broader applicability
MWA / Hurley-Walker
- MWA and LOFAR fully operational
› Theorists, we need you!
- predictions for polarised
synchrotron, Faraday rotation
- mock skies &
data challenges
Gaensler et al. (2011)
- radio surface-brightness
simulations & calculations